Since serine-1981 is phosphorylated after IR and exposure to other agents that induce DSBs [7, 23, 24], and also in human cancers as a consequence of replicative stress [25, 26], and because phosphorylation on ATM serine-1981 can have a very high stoichiometry [7], it is important to determine whether ab32420 identifies either the non-phosphorylated or phosphorylated protein selectively. This evidence concerns the gene ATM and cancer.